This invention relates generally to leak detection methods which are employed with underground tanks for storing gasoline at petroleum products service stations, and tanks in other installations, and more particularly to a leak testing method which permits such a station operator, for instance, to meet the high standards with respect to leak management set by applicable statutes, or by fire marshals, in many states.
Environmental standards in many states today, permit a maximum leakage of no more than a negligible 0.05 gallons per hour from large storage tank holding hazardous liquids. It is conventional for approved operators, using approved leak detection equipment, to periodically check or inspect such storage tanks, which may consist of a 26 foot long cylindrical tank having a diameter of 8 feet, and holding perhaps 10,000 gallons of gasoline. Normally, private equipment operators with portable equipment are employed by station operators and others to periodically test their tanks for leaks. Older methods simply measured any loss in air pressure at the top of the tank but were not sufficiently accurate. Present day leak detectors are typically of the hydrostatic, temperature-compensated variety. One method popularly used is to overfill the tank and monitor air pressure changes (either air bubbling back pressure or regulated pressure, applied to a stand pipe's liquid surface), while at the same time sensing the temperature of the liquid in the tank, because, when the temperature of the liquid changes, the volume of liquid also changes, and it is necessary to compensate for temperature change when this method is used. The problem with seeking to compensate for temperature change in gauging a liquid level differential is that the temperature of the various strata of liquid in the tank differs, so that any temperature reading taken at one location in a tank is not accurate for all of the liquid in the tank. Thus, taking an averaged temperature and correcting for temperature change, creates considerable problems for the testing equipment and may not always provide the accuracy which is deemed necessary for safety. The equipment described in some of the following prior art patents is of this type:
______________________________________ 1,598,571 Fox et al 4,006,636 Hohmen 3,580,055 White 4,186,591 Mooney 3,587,316 Kapteyn 4,300,388 Hansel et al 3,818,752 Lindeberg ______________________________________